Method and apparatus for coating both sides of a moving web

ABSTRACT

A method and apparatus for coating both sides of a moving web, such as a paper web, with the same coating material on both sides by feeding the web downwardly between a rotating support roller and a coating blade directed against said roller. One side of the web is partially wrapped around said roller and a first excess of coating material intended for the web side facing said roller is applied to the roller before or at the point of contact between said web and the roller. A second excess of coating material is applied to the other side of the web by feeding the web through a dam of coating material arranged below said contact point--in the moving direction of the web--said dam being formed substantially between said coating blade and holder and the said other side of the web. The excess coating material is laterally expressed in said dam beneath the top level thereof.

The present invention relates to a method and apparatus for coating bothsides of a moving web, such as a paper web, with the same coatingmaterial on both sides, whereby the web is running downwardly between arotating support roller and a coating blade and holder cooperating withsaid roller.

A simultaneous treatment of both sides of a moving paper web with thesame coating material is known for example in a sizing press, wherebythe paper web is moved vertically downwards through a press nip formedbetween two rotating rollers pressing against one another, the coatingmaterial is applied to both sides of the paper web so that a dam ofappropriate height is formed on either side of the paper web between theweb and the roller on the same side.

When coating in accordance with this known method a number ofdisadvantages occur. In using a starch solution or a pigment dispersionas coating material, an orange-peel effect is obtained in the appliedcoating layer due to the fact that the paper web and the roller surfacesare separated from one another after said nip at largely the same speed,resulting in a dripping in the applied coating layers where the surfacesare removed from one another. This problem increases with increasing webspeed.

Another problem which also increases with increasing web speed is theincidence of considerable turbulence in these dams of coating material.The reason for this is that the boundary surfaces of the dam, i.e. theroller surface and the paper web, have the same direction of movementand thus the coating material is brought into movement downwards towardsthe nip by the respective surface. At high web speeds this downwardlydirected movement becomes very vigorous and results in the flows ofcoating materials being deflected upwards at great force as a result ofa collision when they meet near the bottom of the dam, whereupon theflows reach the surface of the dam. This movement may be so violent thateruptions occur and the coating material is thrown up and out of thedam, thereby limiting the web speed. Thus it has been found that thereis a certain top critical speed above which the sizing press cannot beoperated. This top speed depends on various factors but is regarded asbeing between 400 and 600 meters per minute.

In a further known method for simultaneously coating both sides of apaper web with the same coating material the paper web is caused to passa dam of coating material, when moving in a downward direction, said dambeing limited if seen in the direction of movement of the web by acoating blade which presses said paper web against a rotating supportroller, so that the coating material in the dam comes into contact withboth sides of the web. The advantage of this known method is that iteliminates the above mentioned orange-peel effect.

However, it has been found that this known method--the so calledBILLBLADE® method--in some cases results in a break of the paper webwhen the web passes the coating station, especially if the paper isextremely thin or is of very low strength. The reason for this is thatwhen the paper web passes into the nip between the supporting roller andthe blade it is only supported to a minor degree by the surface of therotating roller. Another disadvantage with this known method occurs atrelatively high web speeds, whereby a considerable movement andagitation arises in the sub-dam formed between the roller and the paperweb, just as in the case of the sizing press, and at higher web speedsthis can also result in the paper web breaking, particularly when thinor weak papers are being coated. It has also been found that the othersub-dam formed between the blade holder containing the blade and thepaper web is much calmer and does not cause any problems, and this maybe due to the fact that in this sub-dam there is only one moving side, ie the paper web, while its other boundary, i e the blade and the bladeholder, is stationary. A calmer circulation thus occurs in the sub-damon this side of the web.

It has now surprisingly been found that the above problems anddifficulties can be eliminated by the present invention.

According to the present invention, a method for applying a coating toboth sides of a web, such as a paper web, is now provided, said methodcomprising the steps of

(a) feeding the web downwardly between a support roller and a coatingblade directed against the roller;

(b) arranging one side of the web to partially wrap around the rotatingroller and applying a first excess of coating material intended for theside of the web facing the roller to said roller before or at the pointof contact between the web and said roller;

(c) applying a second excess of coating material to the other side ofthe web by passing the web through a dam of coating material situatedbelow said contact point--if seen in the direction of movement of theweb--said dam being formed substantially between the coating blade inits holder and said other side of the web; and

(d) pressing the excess coating material out laterally in said dam belowthe top level thereof.

This invention also provides an apparatus for coating both sides of amoving web, such as a paper web, with the same coating material, inwhich the web is moved in a downward direction between a rotatingsupport roller and a coating blade, directed against the roller, theimprovement consisting in a support roller rotating with the web, afirst supply means for coating material arranged above a horizontal line(C--C) through said roller centre, and a coating blade and holderarranged just below said horizontal line (C--C), said blade and holdertogether with the side of the paper web remote from said roller surfaceforms a space for a dam of coating material.

Other features of the invention will be apparent from the details givenin the accompanying claims.

In order that the invention will be more fully understood the followingdescription is given merely by way of example, reference being made tothe accompanying drawings, in which

FIG. 1 is a schematic end view of a known apparatus, a so calledBILLBLADE® device;

FIG. 2 is a schematic view showing the principle of the method accordingto the present invention, and

FIG. 3 is an enlarged portion of a detail of FIG. 2.

In the known BILLBLADE method shown in FIG. 1, a paper web 4 is fedsubstantially vertically downwards--in the direction of the arrowA--into a dam of coating material formed between a support roller 1rotating in the direction of arrow B, and a blade 3 with an associatedblade holder 2. The point of contact between the tip of the blade andthe paper web and the surface of the roll is denoted 5. It is oftenpreferred that this point should be situated somewhat below a horizontalline C--C through the center of the roller. The angle α between theradius from the roll center to the contact point 5 in relation to thehorizontal line C--C through the roll center is about 10°-15° . In thisknown method, therefore, two sub-dams 14 and 15 respectively are formedone on each side of the web of paper.

FIG. 2 shows an embodiment of an arrangement according to the presentinvention. The support roller 1 rotates in the direction of arrow B. Theblade holder 2 with a blade 3 fixed therein is arranged to press a webof paper 7 against said roller 1. A guide roller 9 is arranged to directthe incoming paper web 8 so that it partially encircles said roller1--the wrapping angle is designated β and constitutes the angle betweenthe roller radius to the point 10 and the horizontal line C--C throughthe roller center--i e the web 8 meets said roller at a point 10 abovesaid line C--C through the roller center. An excess amount of coatingmaterial is in a conventional manner supplied to one side of the bladesuitably by means of one or more spray tubes 11, and so that a dam 17 isformed. On the other side of the web, however, the coating material issupplied in excess in the form of a directed stream 16 substantiallytowards the nip formed between the outer surface of the roller 1 and thepart of the paper web which is situated before said tangential point 10.The coating material is advantageously supplied through a tube havingone or more slots 12 so that the coating material is injected into thisnip at high speed and in a substantially coherent stream.

It has been found that the problems discussed above occurring when usingan equipment according to FIG. 1 are completely eliminated by thepresent invention. Thus it has been found that even a very thin weakpaper can be coated on both sides according to the invention without anyrisk of breakage. Due to the fact that the thin or sensitive paperaccording to the present invention is made partially to wrap around theroller surface it is supported by said surface and the rotating surfaceof the roller acts more satisfactorily as a driver and is assisting thefeeding of the paper web towards the nip. The stresses arising in thepaper web just when it is passing through the nip are thus relieved at aplace where of course the friction or braking effect is at a maximum.

It has also been found that the problems in connection with the dam onthe roller side also described above are completely eliminated. Sincethere is no dam above the point where the paper web is brought intocontact with the roller, there are no problems associated withdisturbance in the dam.

According to the embodiment shown in FIG. 2, it is preferred to use atube with a longitudinal slot since in that way it is easier to providea film of coating material which is injected into the nip insubstantially coherent form. In this case the coating material issupplied in excess, which means that the coating material is fed by thefeed means 12 in excess in relation to the quantity of coating materialtaken up by the paper on the side facing the roller.

The excess of coating material is expressed laterally first in aposition below the top of the dam 17 because the pressure conditionscause the paper web to form a bag-like configuration between the roller1 and the paper web 7--which is shown in the form of a broken line 13 inFIG. 2. This bag-like configuration is thus defined by a part of theroller surface and a part of the paper web and consequently bag-likeopenings are formed on each side of the paper web. The excess coatingmaterial is pressed out laterally through these openings and is beingmixed with the excess also discharged laterally from the dam 17. Thesetwo excess amounts of coating material are collected by known means,such as funnel-like means disposed on either side of the two ends of theroller. Since the bag is formed beneath the top of the dam 17, there islargely equal pressure on the inside and outside of said bag andconsequently the paper web is not subjected to any appreciable stresseswhen the surplus on the roller side is pressed out laterally.

It has been found advantageous to vary the wrapping of the paper web 8around the roll 1 according to different coating conditions. Thewrapping angle β may advantageously be as much as 30° and, under certainconditions, for example 15°. It has been found, however, that thewrapping angle β should not be less than 10°.

The supply means 12 need not necessarily consist of one or more tubeswith slots, also other known means may of course be used. The importantfeature is that they cause the coating material to be projected in acontrolled and preferably uniform stream towards the nip between thepaper web 8 and the surface of the roller 1. In some cases it may beadvantageous to direct the jets somewhat downwards against the rollersurface. Otherwise it may be of advantage to direct the stream ofcoating material somewhat upwards against the paper web.

According to an alternative embodiment of the invention, an excess ofcoating material can also be applied to the side of the paper webdirected towards the roll, whereby the coating material is applied inexcess to the roller surface before the contact point 10 between the web8 and the roller surface. This can advantageously be effected by using aform of doctor levelling, whereby a substantially uniform distributionof the applied layer of coating material is also achieved before thecoating material meets the web 8 at the contact point 10. In this casetoo, the excess coating material is pressed out laterally in a positionbeneath the top level of the dam 17 and probably just in front of thecontact point of the blade 3.

It has thus surprisingly been found that the above mentioned problemsarising in coating thin and weak paper webs are completely eliminated bythe present invention at both low and high web speeds, and that verysatisfactory coatings can be obtained.

As an example of application of the present method at very high speeds,sizing can be carried out with a starch solution having a dry substancecontent of 12%, and a web speed of 1000 meters per minute and more. Thesurface weight of the paper has been as low as 40 g/m² and the paper hasbeen practically unsized. The coating result was very satisfactory andthe starch was evenly distributed over both sides of the paper. Therewas not splashing or dripping. At comparative tests in both aconventional sizing press and in a BILLBLADE coater it was foundimpossible to achieve this high speed without very considerablesplashing and interruption problems.

I claim:
 1. A method of coating both sides of a moving paper web byfeeding the web downwardly through coating material contained in a damcomprising a support roller rotating in the same direction as the weband a cooperating blade directed towards the roller and forming with thelatter a nip through which the web passes, one side of the web beingpartially wrapped around the roller beginning at a location above thesurface of the coating material in the dam, characterized in that anexcess of coating material is introduced between the roller and said oneside of the web at about the location where the web begins to wraparound the roller such that on passage of the web through said nip theexcess coating material thus introduced causes a portion of the web tobulge outwardly from the roller in the dam and below the surface of thecoating material contained therein and the excess of coating material tobe discharged laterally into the dam at locations below the surface ofthe coating material contained therein.
 2. A method according to claim 1wherein the excess of coating material is fed to the side of the paperweb facing the roller in the form of a directed stream, substantially ina nip formed between the outer surface of the roller and the part of theweb which is situated in front of and close to the point of contactbetween the web and said roller as the former begins to wrap around thelatter.
 3. A method according to claim 1 wherein the excess of coatingmaterial is applied to the side of the web facing the roller by applyingthe coating material to the roller surface close to and before the pointof contact between the web and the roller as the former begins to wraparound the latter.
 4. A method according to claim 3, wherein the coatingmaterial is applied to the roller surface by doctor means.
 5. A methodaccording to claim 2, wherein the directed stream of coating material isinjected into the nip as a substantially uniform stream and atrelatively high speed.
 6. A method according to claim 2 or claim 5,wherein the coating material stream is supplied at approximately thesame speed as the web.
 7. A method according to claim 6 wherein the webwrapping angle between the radius of the roller to the point of contactwhere the web begins to wrap around the roller and a horizontal linethrough the roller center is at least 10%.
 8. An apparatus for coatingboth sides of a web with coating material comprising a rotatable rollerand an angularly disposed coating blade disposed in proximity to thesurface of the roller to form therewith a dam for containing coatingmaterial and a nip at the lower end of the dam, means for running a webto be coated through said dam and nip with one side partially wrappedaround said roller beginning at a location above the surface of thecoating material in the dam, and means for introducing an excess ofcoating material between said roller and the adjacent side of the web atabout the location where the web begins to wrap around said roller. 9.Apparatus according to claim 8 in which the coating material introducingmeans comprises a slotted tubular member extending axially of the rollerand disposed to discharge coating material uniformly between the rollerand the web.
 10. Apparatus according to claim 8 in which the coatingmaterial introducing means comprises doctor means for applying an excessof coating material to the roller.